Download Chapter 11: The Coast: Beaches and Shoreline Processes

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p347, 3-19
2. In the summertime, waves are usually low-energy waves. These
low- energy waves push sand onshore, so the berm is wider and the beach
face is steeper. In the wintertime, waves are usually high-energy
waves. These high energy waves carry sediment from the beach face
offshore. This beach, then, is narrower with a gentler slope. If
sufficient sediment is moved offshore by winter waves, the wintertime
beach may be rocky.
3. The speed of the longshore current increases with increasing wave
energy (wave height), steeper beach, increasing wave frequency, and
increasing angle between breaker and beach.
4. Longshore drift is sediment transported by the longshore current.
The stronger/faster the longshore current, the more sediment can be
moved. (Just as a faster river can carry more – and larger – sediments)
5. SIMILAR: The flow of water in a stream and in a longshore current
is directional and rather narrow.
DIFFERENT: Rivers flow in one direction (downhill) all the time. A
longshore current’s direction of flow can be reversed depending on the
direction of wave approach.
6. The longshore current reverses its direction because the direction of
wave approach can differ. Along both U.S. coasts, the primary direction
of annual longshore drift is toward the south.
7. Rip currents form when seawater accumulates on a beach and
must, because of gravity, return to the ocean. Rip currents flow
perpendicular to shore and move swiftly through oncoming breakers. The
rip current is usually narrow. A person caught in a rip current should
attempt to swim perpendicular to the current flow (parallel to shore) to get
out of the usually narrow rip current. If the person is unable to do that, he
or she should let the current carry him or her outward until the velocity
lessens.
8. Wave-cut cliffs, sea caves, sea arches, sea stacks, and marine
terraces are all caused by wave erosion along a rocky shore.
- Wave-cut cliffs are formed when waves undercut the lower rocks at
the base of a steep escarpment. The undercutting of the base causes part
of the cliff to collapse.
- Sea caves occur at the base of cliffs where waves erode harder
rocks.
- Continued wave erosion may enlarge caves into channels that run
through a headland, resulting in a sea arch.
- If/when the arch collapses, the headland may be separated into two
parts. The seaward remnant is the sea stack.
- Marine terraces are wave-cut platforms that have been uplifted
above sea level.
9. The following features are made up of sediments deposited by waves
and longshore current.
- A spit is that part of a beach deposit that extends into the open
water of a bay. It often has a curved end due to wave diffraction and tidal
currents. (ex: Provincetown, Cape Cod, MA)
-
If the spit grows faster than currents can keep the mouth of the
bay open (free from sediments), the spit may “bridge” the bay
mouth and close off the bay from the ocean. This then is a bay
barrier or bay-mouth bar.
- A tombolo is a ridge of sand that extends from a beach to an island
or sea stack. (famous example, the beach in Chitty Chitty Bang Bang)
-
Barrier islands are long linear sand ridges that are separated
from the mainland by a lagoon or back bay. (Eastern NC, New
Jersey pictured below) Barrier islands lie parallel to the coast.
10. If sea level rises, barrier islands migrate landward as waves wash
sand over the dunes. Older peat deposits, laid down in back-bay
marshes, are covered by younger sand deposited above. Marsh or peat
deposits are older toward the ocean beach and younger toward the current
salt marsh.
11. Rivers that carry high sediment loads can build deltas once the rivers
debouch (d’BOOSH) into the ocean. If more fluvial (river-carried)
sediment is provided than marine processes can redistribute, a bird’sfoot delta results. (ex Mississippi river delta)_
If marine processes can erode and transport sediments faster than the river
can deliver them, then the delta will have a smooth curved “arcuate”
(curved) shape (ex. Nile river delta)
12. Beach compartments consist of rivers that supply sediment, the
beach where longshore current redistributes sediment, and offshore
submarine canyons that accumulate sediments. If dams are built across
the rivers that provide sediment, the beach will become more and more
narrow and may disappear.
13. Primary coasts are geologically youthful and have features that
are formed by non-marine processes. The subcategories of primary
coasts are listed in Table 11-3 and include the following five main types:
land erosion, subaerial (out on land, not underwater) deposition, volcanic,
coasts shaped by earth movements, and ice coasts.
In contrast, secondary coasts are geologically more mature, and
marine processes (physical or biological) have modified the original
primary coast. The subcategories of secondary coasts are also listed in
Table 11-3 and include the following three main types of secondary
coasts: wave erosion, marine deposition, and coasts formed by biologic
activity (corals, etc)
14. Tectonic changes in sea level are caused by tectonic movements
(uplift or subsidence) of a continent or an ocean basin. These tectonic
changes might be large-scale, caused primarily by plate tectonics
processes, or smaller scale, caused by sediment loading or ice loading.
These tectonic changes are not global, even if they are large scale. For
example, tectonic uplift is occurring along the west coast of the U.S. at the
same time as tectonic subsidence is occurring along the east and Gulf
coasts. In contrast, eustatic changes are global changes in sea level.
They are caused by changes in the volume of seawater (glacial ice buildup
or melting) or by changes in the volume of the ocean basin itself (changes
in spreading rates). Whatever the cause, changes in sea level are
evidenced by features that were once submarine but are now
subaerial. For example wave cut terraces are exposed many feet above
current high tide. Or, features that were once subaerial but are now
submarine, such as “drowned” river valleys.)
Simplification:
Tectonic changes in sea level = CAUSED WHEN LAND MOVES
Eustatic changes = CAUSED WHEN WATER MOVES
15. Coasts advance seaward if land is uplifted or sea level drops.
Coasts retreat if land is submerged or sea level rises.
Actually this is common sense if you visualize the “micro” coastline
movement when the tide floods or ebbs. Or the movement of a single
wave, for that matter!
16. The four main factors that affect whether coasts are erosional or
depositional are erosional features such as wave-cut cliffs, uplift,
depositional features, and subsidence. Uplift and subsidence (sinking,
“subsiding”) may be caused by tectonic forces or isostatic movements. If
uplift occurs, then features shaped by waves will be found above high-tide
levels, for example, uplifted marine terraces. If subsidence occurs, then
longshore transport can build sand bars and sand ridges.
17. Much of the Atlantic coast is subsiding because it is seismically
inactive. Land subsides as ocean lithosphere cools. Some parts are loaded
with sediments and are isostatically subsiding (sinking). Other areas, such
as Maine, are isostatically rising due to glacial rebound. Some areas, such
as the Mississippi Delta, are subsiding because of increased sediment load.
Simplification:
Think of “isostatics” as either rising or sinking because of buoyancy, only
a LOT slower since continental crust flows a LOT more slowly than
water!
18. The Atlantic coast is primarily submergent
(subsiding/sinking/descending, like a submarine) and is characterized by
depositional features such as barrier islands.
19. Types of hard stabilization (structures built to preserve the current
shape of a coast or beach.)
Groin – barrier built perpendicular to coastline, designed to trap sand
and prevent longshore transport.
Jetty – nearly identical to a groin but its purpose is not to trap sand,
but to protect a harbor from waves.
Breakwater – barrier built away from the berm but parallel to the
shoreline
Seawall – barrier built right on the berm to protect property. Not
efficient: they take the brunto of breaking waves (breakwaters are farther
out where the waves are fully breaking yet)
Some parts of the Atlantic coast are emergent (e.g., Maine) and have
uplifted wave-cut features. Glacial moraines form islands, such as Long
Island and Cape Cod. Drowned river valleys such as Chesapeake Bay are
common. Most areas show evidence of erosion.
The Gulf coast is primarily submergent and is characterized by
depositional features such as barrier islands. Some parts of the Gulf
coast are rapidly subsiding (e.g., the Mississippi Delta area,) and these
areas experience the fastest erosion rates.
The Pacific coast is primarily emergent and experiences the least
erosion. Some beaches are becoming narrower through time because of
dams across rivers.
19. Groins and jetties are similar in that these are structures built
perpendicular to the shoreline.
The primary purpose of groins is to trap sediment between adjacent
groins.
Jetties also trap sediment, but the main purpose of a jetty is to protect
a harbor entrance.
A breakwater is a structure built offshore parallel to the coast. The
aim of a breakwater is to protect a harbor from more intense wave activity.
An unwanted consequence of a breakwater is that sediments are deposited
in the harbor.
A seawall is built on sand to protect the beach from erosion. In most
cases, seawalls do not work well or long. They are frequently undermined
by increased wave action on the sea side, so that the wall collapses. An
unintended consequence of a seawall is a narrowing of the beach